I'm on my mother-in-law's PC. She has Windows Vista 32-bit. I had OpenTTD 1.1.3 AND TTD for Windows on the same machine. Both were running just fine. I tried to update OpenTTD to version 1.2.3. I accidentally installed it to the wrong folder.
This is when IT all "hit the fan". 
As of this point, version 1.2.3 seems to be O.K. but I haven't been able to get TTD for Windows to run again. I continue to receive error messages: "Unable to access Transport Tycoon Deluxe CD!$"
I'll be leaving in a little while and I won't be back for some time. I live too far away to pop on over and fix it. It has to be done NOW!!My copy of "TTD4Win" was downloaded from a link on the Forums (i.e. here) some time ago along with the TTDPatch and every tool I could find. I have no more time for the "trial and error" method.
NOTE: None of this may matter now. I discovered a short time ago that the folder which contained all the material I had spent all night downloading had been permanently deleted.
MORAL: Never use someone else's stuff. Esp: Their PC.